The present disclosure relates to a fixing device used for an electrographic image forming apparatus, and an electrographic image forming apparatus.
The electrographic image forming apparatus includes a fixing device configured to fix a toner image transferred from a photosensitive drum onto a sheet. In the fixing device, a pressing roller is pressed onto a fixing roller heated by a heat source such as a halogen heater, the sheet passes through a nip part formed between the fixing roller and the pressing roller, and the toner image formed on the sheet is heated and pressurized, thereby being fixed on the sheet. It is to be noted that, in recent years, instead of the fixing roller, a fixing device using an endless fixing belt has also been developed.
In such a fixing device, when a sheet on which a toner image is formed passes through the nip part, there is a tendency that the sheet winds on a circumference face of the fixing roller due to viscosity of melted toner. In order to prevent such a winding of the sheet onto the fixing roller, in the fixing device, a separating plate for separating the sheet that passed through the nip part from the circumference face of the fixing roller is provided. The separating plate is formed in a rectangular plate, and is placed at a downstream side of the nip part. Long sides of the separating plate extends in parallel with an axis of the fixing roller and short sides of the separating plate extends in a sheet conveyance direction. In addition, in the separating plate, an edge part facing the nip part is extremely close to the circumference face of the fixing roller, but is not in contact with the circumference face of the fixing roller, and is apart from the circumference face of the fixing roller at a predetermined distance.
As described above, in the fixing device, the fixing roller and the pressing roller are pressed one another. The circumference face of the pressing roller is pressed by the fixing roller and is subject to elastic deformation partially in a recessed shape. Apart between a part that presses the pressing roller in the fixing roller and a part that is pressed by the fixing roller and is subject to elastic deformation partially in a recessed shape in the pressing roller corresponds to the nip part. It is to be noted that, among types of the fixing devices, there is a fixing device in which the fixing roller (not pressing roller) is subject to elastic deformation in a recessed shape and a fixing device in which both of the fixing roller and the pressing roller are subject to elastic deformation. However, here, a case where the pressing roller is subject to elastic deformation in a recessed shape is exemplified.
When the force for pressing the fixing roller and the pressing roller one another increases, the degree that the fixing roller eats into the pressing roller increases, and the degree that the pressing roller is subject to elastic deformation in a recessed shape increases. As a result, the length of the nip part in the sheet conveyance direction is lengthened. When the nip part is lengthened, the degree that the sheet that passes through the nip part is curved along a recessed shape of the nip part is enlarged, and the ejecting direction of the sheet that is ejected from the nip part comes close to the circumference face of the fixing roller.
When the degree that the ejecting direction of the sheet that is ejected from the nip part comes close to the circumference face of the fixing roller is enlarged, the end part of the sheet that is ejected from the nip part comes into contact with an end surface facing the nip part in the separating plate and is largely bent, and then, the sheet is sometimes out of the conveying path. As a result, jamming is sometimes caused inside the fixing device.
Accordingly, in the fixing device, while considering stability of fixing of a toner image, in order to prevent the ejecting direction of the sheet that is ejected from the nip part from coming close to the circumference face of the fixing roller, the force for pressing the fixing roller and the pressing roller one another is appropriately set, thereby ensuring an appropriate length of the nip part.
However, depending on the usage, such as a case where a long-time continuous printing is carried out, of the image forming apparatus, in use, the hardness of the pressing roller is decreased. As a result, the pressing roller is pressed by the fixing roller, the degree that the pressing roller is subject to elastic deformation in a recessed shape increases, and the nip part is lengthened. In addition, when the nip part is lengthened, the degree that the ejecting direction of the sheet that is ejected from the nip part comes close to the circumference face of the fixing roller increases, and thus there is an increased risk that jamming is caused inside the fixing device.
In order to prevent such a generation of jamming, in a fixing device or an image forming apparatus, it is favorable that, for example, an increase of degree that the ejecting direction of the sheet ejected from the nip part comes close to the circumference face of the fixing roller is recognized, and control for avoiding a generation of jamming is automatically carried out based on the recognition.
However, the degree that the ejecting direction of the sheet ejected from the nip part comes close to the fixing roller less changes, and therefore, the change cannot be easily recognized.
On this point, a technique is known in which an abnormal trace of the sheet is detected by a transmission optical sensor that allows light to be transmitted toward the axis direction at the downstream side of the fixing roller. However, it is difficult to detect a small change of the ejecting direction of the sheet ejected from the nip part by this technique.
It is to be noted that, immediately before generation of the jamming in the fixing device, the sheet begins to be congested at the downstream side of the fixing roller, the trace of the sheet ejected from the nip part largely changes. According to the above-mentioned technique in which the abnormal trace of the sheet is detected by the transmission optical sensor, there is a possibility that such a large change of the trace of the sheet is recognized. However, even if the large change of the trace of the sheet immediately before the jamming is generated is recognized and control to avoid the jamming can be initiated, the sheet is already beginning to be congested at the downstream side of the fixing roller, and thus, actually, generation of the jamming is sometimes not capable of being avoided.